Sputtering holes with ion beamlets

ABSTRACT

ION BEAMLETS OF PREDETERMINED CONFIGURATIONS ARE FORMED BY SHAPED APERTURES IN THE SCREEN GRID OF AN ION THRUSTER HAVING A DOUBLE GRID ACCELERATOR SYSTEM. A PLATE IS PLACED DOWNSTREAM FROM THEE SCREEN GRID HOLES AND ATTACHED TO THE ACCELERATOR GRID. WHEN THE ION THRUSTER IS OPERATED HOLES HAVING THE CONFIGURATION OF THE BEAMLETS FORMED BY THE SCREEN GRID ARE SPUTTERED THROUGH THE PLATE AT THE ACCELERATOR GRID.

"United States Patent Othce 3,826,729 Patented July 30, 1974 ABSTRACT OF THE DISCLOSURE Ion beamlets of predetermined configurations are formed by shaped apertures in the screen grid of an ion thruster having a double grid accelerator system. A plate is placed downstream from the screen grid holes and attached to the accelerator grid. When the ion thruster is operated holes having the configuration of the beamlets formed by the screen grid are sputtered through the plate at the accelerator grid.

ORIGIN OF THE INVENTION The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention is concerned with forming holes of various shapes. The invention is particularly directed to sputtering holes with ion beamlets. The invention also relates to the correlation between screen grid hole configuration and ion beamlet shape in the plane of the accelerator grid of an ion thruster.

Electron-bombardment ion thrusters of the type described in U.S. Pat. Nos. 3,156,090 and 3,324,659 are being considered for a variety of space missions for which the optimum specific impulse is between about 2,000 and 3,000 seconds. As the specific impulse decreases, the discharge energy required per beam ion, eV/ion, becomes an increasingly important loss mechanism. The eV/ion decreases as the open area fraction of the screen grid increases.

The standard thruster grid configuration has consisted of a hexagonal array of circular holes in both the screen and accelerator grids as shown in U.S. Pat. Nos. 3,238,- 715 and 3,262,262. The maximum open area fraction for a hexagonal array of circular holes is 0.906, which occurs when the minimum web distance is zero. Larger open are fractions can be obtained with arrays of hexagonal, square, or triangular holes if the Web thicknesses are made equal to that of the hexagonal arrays of circular holes. Grids with such hole shapes would provide improved discharge performance.

In addition, electrostatic beam deflection concepts have been proposed for thrusters sized for satellite station-keep ing application. By way of example, vector grids having square apertures have been used. It is contemplated that screen grid hole shapes other than circular may provide optimum beam deflection characteristics.

SUMMARY OF THE INVENTION Holes of various shapes are ion sputtered in accordance with the present invention. An electron-bombardment ion thruster with a double grid accelerator system has a plate in which holes are to be formed attached to the accelerator grid. This plate is positioned in front of the screen grid holes having the desired configuration.

During thruster operation holes are sputtered through this plate. A correlation between screen grid holes shapes and ion beamlet shape in the plane of the accelerator grid may be made from information obtained.

OBJECTS OF THE INVENTION It is, therefore, an object of the present invention to provide an improved process for sputtering holes of a predetermined configuration through metal plates.

Another object of the present invention is to provide ion beamlets having a predetermined configuration for ion sputtering.

A further object of the invention is to increase the open area fraction of a screen grid of an ion thruster.

These and other objects and advantages of the invention Will be apparent from the specification which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENT Sheets having various shaped holes were used with a 30-cm. thruster of the electron bombardment type to illustrate the features of the present invention. The thruster had a double grid accelerator system including a screen grid and an accelerator grid. The screen grid was 1.52 mm. thick with 5.0 mm. holes drilled on a 5.6 mm. centerto-center spacing. The accelerator grid was 2.0 mm. thick with 4.0 mm. diameter holes.

Various screen grid hole shapes were electric-discharge machined into 0.13 mm. thick tantalum sample sheets. Each sheet had a diameter of about 3.8 cm. The number of holes machined into each sample sheet was such as to surround the central hole with holes in a regular pattern.

The tantalum sheets were placed on the upstream side or triangular holes if the web thicknesses are made equal hole in each tantalum sheet was at a radius of 7 cm. from the thruster axis.

Solid tantalum sheets between 0.5 and 0.13 mm. thick were then placed on the upstream side of four matching areas cut into the accelerator grid. Ions extracted by the holes in the screen sample plate then impinged on the sheets on the accelerator grid and sputtered through in about one-half hour. The shapes of the holes sputtered through the accelerator grid tantalum sheets had the same configuration as the, corresponding holes in the sample sheets mounted on the screen grid. The shapes of the hole patterns eroded on the accelerator grid sheets were representative of the focusing characteristics of the corresponding screen grid holes.

All the holes were ion sputtered in a 1.5 mm. diameter, 4.5 mm. long, vacuum facility. The operating pressures were about 7X10 torr during sputtering. Some thruster operating parameters during the sputtering are shown in Table I.

The length, L, given on Table I was the measured gridto-grid spacing plus the screen grid hole sample sheet thickness. The grid spacing was measured before and after thuster operation and agreed within about 7%.

TABLE L-THRUSTER OPERATING PARAMETERS Screen Accelerator Spacing, V A V L, mm.

0. 42 870 0. 010 3. 52:1;0. 3 54 1, 220 0.010 4. 15:0. 3 .75 1,330 .011 4.1i0.3 40 1, 330 006 4. 1&0. 3 55 1, 330 005 4. 1:1;0. 3

The ion focusing characteristics of circular, hexagonal, square and triangular holes were studied. The geometries of screen and accelerator holes are shown in Table II.

TABLE II.-GEOMETRY OF SCREEN AND ACCELERATOR HOLES Rs, D5, 95, Test Screenholegeometry mm. mm. RA/RS DA/DS deg. deg. :5

2. 5 2. 5 0. 45 o. 45 180 180 2. 5 2. 5 50 .50 180 180 2. 5 2. 8 57 64 120 55 2.5 3.5 .49 .55 90 4a 2. 9 5. 7 55 .91 50 33mm 2. 5 2. 5 .54 64 180 180 2.5 2.8 .55 .52 120 74 2.5 3.5 .51 .74 so 49 Triangle 2.9 5.7 .66 .91 60 27:1:10 4..;.: Curvesidedsquare 2.3 2.5 .57 .57 1394a 79 2.2 2.5 .51 .54 134155 75 2.4 2.9 .54 .75 136:1:3 78 2.3 2.5 .53 .64 l21i3 67 The erosion of holes on the accelerator sample plates dlrectlng said 1011 beamlets toward said members to was observed visually during the tests. The locations of maximum erosion within each beamlet then could be determined because these locations would erode through first and were illuminated by the discharge plasma. In addition, the small areas of accelerator grid samples 0pposite the holes in the screen grid sample holes would fall 01f during the test and areas of erosion could be de termined after the test.

Circular, hexagonal, square and triangular holes were tested simultaneously in tests 2 and 3. Two characteristic dimensions were selected to specify the erosion ratios for the noncircular shapes. The distance from the sample center of symmetry to a side is specified as R or R on the screen or accelerator sample, respectively. The distance from the center of symmetry to the intersection of the sample corners is called D and D The characteristic dimensions of the hexagonal and square samples were about the same as the circular sample. The triangular holes were made somewhat larger than the other samples.

The relation between the angles in the screen holes and the angles of the eroded accelerator grid samples was determined. These angles are described as 0 and 0 respectively. The value of 0 is taken to be that of the fringe area, and in some cases it is difficult to specify exactly due the difficulty of or determining the fringe edges. The values of 0 and 0 are shown in Table II along with the estimated uncertainty of measurement. As indicated in Table II the uncertainty was estimated to be five degrees except where otherwise shown.

While the preferred method has been described it will be appreciated that various modifications may be made without departing from the spirit of the invention or the scope of the subjoined claims.

What is claimed is:

1. A method of producing apertures having predetermined configurations in members comprising the steps of forming ion beamlets having said predetermined configurations by extracting ions from a source through a sheet having at least one aperture therein contoured to said predetermined configuration, and

sputter the same whereby apertures are sputtered therein having the configurations of said ion beamlets.

2. A method of producing apertures as claimed in claim 1 including the step of mounting said members normal to said ion beamlets.

3. A method of producing apertures as claimed in claim 2 wherein the ion beamlets are formed in an ion thruster.

4. A method of producing apertures as claimed in claim 3 wherein the ion thruster has an accelerator grid, including the step of mounting said members on said accelerator grid.

5. A method of producing apertures as claimed in claim 4 wherein the members are mounted in an upstream side of said accelerator grid.

6. A method of producing apertures as claimed in claim 4 wherein the ion beamlets are formed by a screen having apertures therein, said apertures having said predetermined configurations of said ion beamlets.

7. A method of producing apertures as claimed in claim 6 wherein the ion thruster has a screen grid located upstream of an accelerator grid, said screen being mounted on said screen grid.

8. A method as claimed in claim 7 wherein the screen is mounted on the upstream surface of the screen grid.

9. A method as claimed in claim 1 wherein the ion beamlets are formed in a vacuum.

10. A method as claimed in claim 9 wherein the vacuum has a pressure of about 7 X 10- torr.

References Cited UNITED STATES PATENTS 3,576,729 4/1971 Sigournay et al. 204-298 X 3,133,874 5/1964 Morris 204298 3,472,751 10/1969 King 204-192 3,708,418 1/ 1973 Quinn 204--298 JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner Po-w UNITED STATES PATENT OFFiC Q @MTIFIGATE or CRBLECTION Patent No. 3, ,7 9 Dated July 30, \97h Inventor) David C. Byers and Bruce A, B k

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1 i ine 51 cancel "ere" and insert therefor -'-area--.

Column 2, I ine 3 4,, cancel "or triangular hoIes if the web thicknesses are made equal" and insert therefor of the screen grid over four large open areas.

The central",

Signed and sealed this 5th day of November 1974.

(SEAL) Attem: a

McGOY M, GIBSON JR, Co MARSHALL DANN Atteht'ing Officer Commissioner of Patents 

